Measuring vitamin C in fruit using a microscale titration | 14–16 years

They then compile a full set of class results, before planning an investigation to explore how vitamin C levels are affected by different conditions.

Learning objectives

1. Safely carry out a simple microscale titration reaction.
2. Make inferences from ecperimantal data and compare tha mount of vitamin C levels present in different fruits.
3. Evaluate the results of the practical in terms of errors.
4. Plan an experiment to determine how vitamin C levels are affected by different conditions.

The practical allows learners to safely carry out microscale titration reactions and make inferences about the amount of vitamin C present in fruit and juices (LO1 and 2). Completion of the follow-up questions will allow learners to succeed in LO2 and 3. LO4 relates to planning an experiment as a group, which learners can carry out during a later session.

Note: learners may be unfamiliar with the word ‘inferences’ used in the learning objectives. Explain this as ‘a conclusion reached using the evidence collected from a practical and your scientific knowledge’.

Introduction

Ascorbic acid, the compound we call vitamin C, dissolves in water. It is easy to take into our bodies via a solution of the compound, such as that found in fresh fruit juice. It is a key nutrient required in the body for wound healing and iron absorption. It also acts as an antioxidant to protect cells and helps support the immune system to fight infection. A deficiency in vitamin C can cause scurvy.

In this practical activity, learners work in groups to analyse different samples of fresh fruits and juices, using a simple microscale titration with iodine to determine the amount of vitamin C present. When ascorbic acid reacts with iodine, dehydroascorbic acid and hydrogen iodide are formed. Iodine is brown in colour, but the ions of iodide in the product are colourless.

By adding starch to the solution, the colour change observed is more distinct as the iodine and starch molecules combine to make a blue−black complex. This allows learners to determine when all the iodine has reacted with the ascorbic acid, as when all the iodine has reacted with ascorbic acid, there will be no blue−black complex left. This is called the endpoint of the reaction.

Scaffolding

• Two versions of the worksheet are available: scaffolded (one star) and unscaffolded (two stars). The different sheets are indicated by the number of stars in the header.
• The scaffolded sheet offers more support to allow learners to access the questions. Hints are found after some of the questions to support learners further and guide their answers.  

Teaching sequence

Introduction

1. Invite suggestions about where vitamin C comes from, together with ideas about how much vitamin C is present in the source suggested.
2. Organise suggestions in a primitive table – ‘lots’, ‘some’ and ‘little vitamin C’.
3. Introduce the practical, to test different fruits and juices for their vitamin C levels, and the learning objectives. Explain that the test will confirm or change some of the suggestions made.

Testing samples to determine vitamin C levels

1. Organise learners into groups of three or four, with four samples to test per group. Note: the same fruit or juice can be tested more than once by different groups, giving multiple results for reliability.
2. Demonstrate the technique for testing the samples.
3. Circulate and supervise the groups as they extract juice from fresh fruit and test the juice samples.
4. Collate the results.

Example results table

Method

A written method can be found in the student sheets and in the lesson slides.

Technician notes

Read our standard health and safety guidance and carry out a risk assessment before running any live practical.

Further preparation and guidance are given in the technician notes section of the teacher guidance.

Reviewing the results

When the groups have completed their tests, review their results. In a class discussion, review all the results and:

• Use any duplicate results to help learners understand about the reliability of test results.
• Refer to juice packaging materials to give background information.

Investigating conditions affecting vitamin C levels

1. Introduce the next activity in which learners plan an investigation to explore how vitamin C levels are affected by different conditions.
2. Give each group a different investigation to plan, using the basic test as a starting point. Suggested investigations include:
   • How does temperature affect the amount of vitamin C?
   • How does light affect the amount of vitamin C?
   • How does packaging and storage surroundings affect the amount of vitamin C?
   • How does a ‘slow release’ vitamin C tablet work?
   • Do fruits of different varieties (eg mandarin oranges, navel, clementines, satsumas, etc.) contain different amounts of vitamin C?
   • Does frozen, concentrated orange juice contain as much vitamin C as fresh juice?
   • Which fruit contains the most vitamin C per 100 g? Make a thorough comparison of as wide a range of fruit varieties as possible.
3. Agree criteria for peer assessment of investigation plans.
4. Give each group access to information sources and presentation materials.
5. Circulate and support as learners:
   • Plan their investigations.
   • Prepare a presentation of their plan.
   • Elect a spokesperson to feedback to the class.

Plenary

In a plenary:

• Hear each presentation.
• Learners give feedback to each group based on the quality of the proposed investigations and their scientific content.
• Make sure that each learner makes a written record of the planned investigation.

Learners can carry out their investigations in a later session or in an after-school club.

Commentary

Working together on a simple titration gives a sound basis from which groups work in teams to plan an investigation. The suggested investigations permit differentiation by task and outcome.

Agreeing the criteria and then assessing each other’s investigation plans stimulates learners to perform well. The presentations also give an opportunity to evaluate activities across the class, adding coherence and depth to scientific understanding of the issue.

Teacher feedback after the practical work can point out the key scientific features for the investigation plans. Feedback on the presentations will reinforce the peer assessments. Feedback on written work supports development of scientific thinking.

Answers

Answers to both the scaffolded and unscaffolded student sheets can be found in the teacher notes.